1. Field of the Invention
The present invention relates to novel anhydrous crystals of 4-carbamoyl-1-.beta.-D-ribofuranosyl imidazolium-5-olate which can easily be prepared and have an excellent stability against heat and water.
2. Description of the Background Art
4-Carbamoyl-1-.beta.-D-ribofuranosyl imidazolium-5-olate (hereinafter referred to as mizolibine) is a nucleic acid-related compound which has been discovered in a culture broth of Eupenicillium brefeldianum M-2116 (FERM P-1104). It is a weakly acidic substance which is readily soluble in water and decomposes producing brown foam at about 200.degree. C. Various processes are known for producing mizolibine, e.g. J. Antibiotics, 27, (10) 775 (1974), Chem. Pharm. Bull., 23, 245 (1975), Japanese Patent Laid-open Nos. 56894/1973, 1693/1976, 121275/1975, 121276/1975, and the like. All known processes produce mizolibine mono-hydrate.
Mizolibine possesses superior immuno-suppressing activity. The compound is thus widely used in clinics for suppressing denial reactions in kidney transplant and the like, and is sold under the trade mark of Bredinin (product of Toyo Jozo Co., Ltd.). Usually, the medicine is admininistered in an amount of 2-3 mg/kg/day as an initial dose and 1-2 mg/kg/day as a maintenance dose.
There are known two types of mizolibine crystals, monohydrate crystals and anhydrous crystals. Mono-hydrate crystals contain about 6-7% by weight of water (theoretical amount 6.5%). The IR absorption spectrum of mono-hydrate crystals measured by a double beam IR spectrophotometer (KBr method) indicates absorption peaks in the neighborhoods of 3420, 3130, 2925, 2770, 1625, 1540, 1445, 1300, 1260, 1195, 1100, 1080, 1055, 1030, 980, 873, 829, 770, 740, 725, 560 cm.sup.-1 [J. Antibiotics, 27, (10), 775 (1974), Japanese Patent Laid-open No. 56894/1973].
The IR absorption spectrum of mono-hydrate crystals measured by an IR spectrophotometer of the Fourier transform type (interference method) has absorption peaks in the neighborhoods of 3422, 3323, 3122, 2947, 2913, 1689, 1617, 1548, 1444, 1385, 1297, 1208, 1154, 1140, 1107, 1080, 1062, 1035, 982, 946, 874, 843, 824, 779, 746, 724, 679, 627, 567, 482 cm.sup.-1 (Standard deviation: .+-.2 cm.sup.-1 ; FIG. 3). Anhydrous crystals which have conventionally been available (hereinafter referred to as "Anhydrous Crystals A") contain about 0.5% by weight of water. Their IR absorption spectrum measured by an interference-type IR spectrophotometer has absorption peaks in the neighborhoods of 3469, 3348, 3292, 3139, 3018, 2943, 2874, 1654, 1621, 1593, 1542, 1438, 1371, 1347, 1328, 1311, 1282, 1249, 1213, 1188, 1130, 1103, 1058, 1027, 978, 947, 865, 827, 777, 768, 749, 668, 645, 630, 603, 571, and 501 cm.sup.-1 (FIG. 2).
Crystals of mizolibine monohydrate separated by conventional culture methods and chemical synthesis are, however, unstable at a higher temperature or under highly humid conditions. For instance, if mizolibine monohydrate crystals are left to stand at 65.degree. C. in a sealed vial, they become colored to dark green and solidifies in 24 hours. Also, when mizolibine monohydrate crystals which have been dried to dehydration in vacuum at 40.degree. C. for 48 hours in the presence of phosphorous pentoxide are placed in an open vial at 20.degree. C. at 95% RH, the water content are restored to 6.5% which is near the theoretical water content value of the mizolibine crystals at 24 hours. A continued storage for a further 7 days makes the crystals colored to light yellowish green. Because of such unstable nature, mizolibine monohydrate crystals are unsuitable for making a medicinal preparation therefrom, and thus no such preparation is not sold in the market.
On the other hand, Anhydrous Crystals A is more stable than mono-hydrate crystals, and is thus formulated into the commercially available Bredinin tablets. Anhydrous Crystals A, however, are obtained by a complicated process, in which the crystals are deposited from a methanol medium using a small amount of Anhydrous Crystals A as seed crystals. When manufactured, the solid crystals tend to attach to the surfaces of manufacturing units and equipment. Removing the solidified crystals by washing involves a difficult task. In addition, methanol used as a medium must be sufficiently removed.
There have been no reports published concerning the polymorphism of anhydrous crystals of mizolibine.
As a result of extensive studies aiming to solve the above-mentioned problems about Anhydrous Crystals A, the present inventors have quite unexpectedly found that there was a crystal form of mizolibine other than those of monohydrate crystals and Anhydrous Crystals A. The inventors found further that this novel crystal form was extremely stable against heat and moisture, and could be produced using ethanol as a medium. In addition, the newly found crystals had an advantage in that no crystals solidify and attach to the surfaces of manufacturing apparatus and equipment, thus ensuring an easy operation for washing them. Such findings have led to the completion of the present invention.